Monitoring and Modelling Emissions of UK road transport
Executive Summary of Lester Kwiatkowski’s MSc Thesis
Centre for Environmental Policy, Imperial College London
Academic year: 2008-2009
Supervisor: Professor Helen ApSimon
Four main Objectives were identified:
• Construct a dataset of continuous CO2 monitoring at the Marylebone Road air quality
station and collect CO2 and NOx recordings made by the London Air Quality Network (LAQN)
in West Kensington and the Imperial College group located at Silwood Park in Berkshire.
• Assess the extent to which road transport CO2 predictions produced by road transport
models are valid in the light of empirical evidence gathered across the monitoring sites.
• Evaluate the extent that concentration trends and diurnal profiles across the monitoring
sites can be explained by local topography, anthropogenic emissions, biogenic factors and
• Model the potential impact of hybrid buses and electric vehicles on emissions and ground
level concentrations of GHGs and air quality pollutants both across the UK and specifically
within London, accounting for any increased emissions due to electricity generation
The accelerated release of greenhouse gases (GHGs) due to anthropogenic activity is now generally
accepted as the major driver of global warming). In the EU25 countries, approximately 28% of the
energy available for consumption is attributed to transportation, of which more than 80% is due to
road transport. Worldwide, transportation accounts for 27% of primary energy use and is also the
fastest growing sector. The transport sector thus provides a significant target for reducing GHG and
air quality pollutant emissions.
Across the UK there is a legislative requirement for monitoring of air quality pollutant
concentrations. However, this is not the case for GHGs which are typically only monitored on an ad
hoc basis. Thus at present there is a fairly limited understanding of the variety of factors that interact
to influence ground level concentrations of atmospheric species, especially GHGs.
Within the road transport sector there is a great diversity of abatement opportunities that have the
potential to reduce GHGs and air pollutants. Such measures include technical options that affect
both drivetrain (e.g. EVs, HEVs,) and non-drivetrain vehicle performance (e.g. alternative fuels, end
of pipe filters), as well as non-technical options such as fiscal measures, and traffic demand
management. Of all potential abatement options, the greatest publicity has surrounded hybrid
electric vehicles (HEVs) fully electric vehicles (EVs) and H2 fuel cell vehicles. This is in large part
because such technologies represent a radical shift away from a road transport sector dominated by
internal combustion vehicle technology but also because the potential of such technologies in
reducing the environmental impact of the transport sector is considerable.
Throughout the experimental period (March-July 2009) CO2 and NOx data were recorded from three
monitoring station sites that form an urban-rural transect across the Greater London area. External
filters were changed and calibrations performed every 2-3 weeks to ensure data accuracy, and all
outputs were digitally logged at regular intervals.
Concentrations of monitored gases are highly dependent on meteorological conditions. Thus in
order to analyse the compiled CO2 and NOx datasets, concurrent meteorological data was required.
As a result of road canyon-induced wind vortices, standard measurement of wind speed and
direction within the MR street canyon is largely unrepresentative of synoptic wind conditions.
Consequently, wind speed and direction data was taken from the Met Office’s Heathrow monitoring
station as well as at the monitoring sites.
There are two main approaches to modelling emissions from road transport. The present work
utilises a bottom-up methodology. That is, one which is traffic-based, in-line with the NAEI
methodology for air quality pollutants. The traffic based approach adopted in the present study
utilises several datasets. Underlying the BRUTAL model are vector based GIS maps of the UK road
network (all motorways, major roads and some minor roads) coupled with gridded population data.
For any road link there are assigned vehicle flows and speeds coupled with a given vehicle mix for
that road type and associated emissions factors. The combination of these datasets allows the
derivation of emissions from any road link. By altering the vehicle mix and emission factors within
the iMOVE model, the impact of HEV and EV vehicle uptake was simulated up to 2020.
Figure 1. Flow diagram illustrating the nesting of models that contribute to the BRUTAL air quality
and GHG output utilised in the present study.
Results suggested that the factors governing concentration trends differed across the three
• Average CO2 and NOx concentrations were found to vary across the rural, urban background
and urban roadside monitoring locations in line with expectations. The areas of higher
monitored concentrations corresponded to locations with greater road transport activity,
the presence of street canyons and higher emissions from non-transport sources such as
commercial buildings and industry.
• The diurnal cycles described at the rural monitoring site appear primarily a consequence of
daily meteorological conditions and are consistent with the presence of a daily mixing depth
cycle. This is contrasted by the cycles described for the urban roadside location where
mixing depth effects appear largely masked by the effect of daily cycles in traffic volumes.
Finally, the daily concentration cycles described for the urban background site are indicative
of both meteorological and anthropogenic factors exerting influence on ground level
• The significance of both wind speed and direction to pollutant dispersion and ground level
concentrations is evident across all three monitoring locations. This is especially apparent at
the urban street canyon site of Marylebone road where synoptic wind speed direction
appears to cause vortices within the canyon resulting in concentration enrichment on the
• In terms of future transport scenarios, introduction of both hybrid buses and electric
vehicles was shown to significantly reduce road transport tailpipe emissions of both air
quality pollutants and greenhouse gases. As such, each technology option appears to
represent a “win-win” scenario. In addition, even accounting for emissions incurred during
electricity generation and assuming a highly pessimistic coal-based generation mix, EV
uptake still represents a highly beneficial transport scenario.
Discussion, Conclusions & Policy Implications
Across all three monitoring sites there is evidence of ground level emissions of CO2 and NOx. Such
emissions explain why diurnal mixing depth cycles affect monitored ground level concentrations and
why higher wind speeds were also found to reduce ground level concentrations. However what
remains relatively unclear is the source of such emissions. Ground level NOx concentrations appear
to be adequately explained by emissions from the road transport sector and other UK sources,
however this does not appear to be the case for CO2. In the opinion of this author interpretation of
ground level CO2 enhancement therefore requires considerable further study. Specifically, there is a
need for greater understanding of non-road transport emission sources and sinks, and the relative
contributions of both anthropogenic and biological diurnal profiles to ground level CO2. Clearly both
anthropogenic and biological factors are likely to exhibit seasonal and annual variation alongside
spatially variable effects. Thus perhaps the greatest insight to emerge from the present study is the
inadequacy of current GHG monitoring across the UK. In contrast to the regulatory monitoring of air
quality pollutants, the lack of monitoring of GHGs is constraining our understanding of the many
variables which influence their atmospheric dispersion characteristics and resulting ground level
The current study’s aim to model present concentrations of CO2 resulting from road transport has
furthered our understanding of how the BRUTAL/iMOVE models can be manipulated. When
comparing sites in the same vicinity, the BRUTAL/iMOVE models appear to explain much of the
difference between average monitored CO2 concentrations at roadside and non-roadside sites.
However this approach of modelling CO2 concentration enhancement from road transport fails to
explain, perhaps expectedly, the difference that exists between average monitored CO2
concentrations at urban and rural sites. To fully ascertain why this is, further study is required.
Specifically, more CO2 monitoring stations are required and ideally all average monitored CO2
concentrations should be produced for annual periods as opposed to the 2-3 month monitoring
period adopted in the present study.
Finally, in line with the objective of understanding the role of the road transport sector in influencing
ground level CO2 and NOx concentrations, the BRUTAL and iMOVE models were utilised to model the
impact of future transport scenarios on emissions of both GHGs and air quality pollutants. The
findings of this modelling strongly indicate that technical abatement options such as uptake of
hybrid-electric and fully electric vehicles have a crucial role to play in mitigating road transport
emissions in both urban environments and throughout the UK. However, as well as assessment of
tailpipe emissions and calculations of emissions associated with electricity generation such as those
presented within this study, determination of the full benefits and costs of emerging vehicle types
requires thorough Life Cycle Assessment (LCA). Specific areas that may require inclusion into further
LCA analysis include the emissions associated with mining and processing of materials unique to
hybrid and fully electric vehicles. Finally, with regard to electric vehicle uptake across the UK, greater
research is required on how vehicle charging profiles could affect requirements for additional
electricity generation, and the potential to influence when vehicles are charged through “smart
charging” and policy implementation.
In terms of future transport scenarios, a summary of policy recommendations is outlined below:
• Subject to a thorough LCA comparison of hybrid and conventional IC buses, nationwide
replacement of IC buses with diesel electric hybrids is recommended.
• The benefits of hybrid buses are greatest in urban areas especially inner cities yet uptake
rates are extremely low outside London. Greater financial support is likely to be required to
facilitate a nationwide transition to hybrid buses given the higher capital investment such
• Given the rate of technology development in low carbon vehicles, the higher capital costs of
hybrids and the relatively minimal reductions in GHGs and air quality emissions they confer,
a phased introduction of hybrid buses, as proposed for London, is probably advisable
• Replacement of conventional IC vehicles with EVs offers substantial GHG and air quality
reductions to road transport emissions but in the short term such vehicles are likely to be
constrained to urban areas. It is proposed that all major UK cities develop plans of how they
intend to facilitate and incentivise EV uptake.
• The benefits that EVs confer over conventional IC vehicles are progressively greater as UK
power generation shifts towards less carbon intensive and more sustainable sources of
electricity generation. Although a certain number of EVs may be accommodated by greater
utilisation of electricity from periods of low demand, widespread uptake of EVs in cities
other than London is likely to require greater UK electricity production.